1. Field of the Invention
This invention relates to a method and system for canceling jamming signals utilized as a countermeasure against radar transceivers and the like and is therefore a countermeasure for the countermeasure.
2. Brief Description of the Prior Art
Improved jammer capability and ultra-sensitive radars have brought on a new electronic counter measure (ECM) threat. This threat occurs when a high powered jammer uses its directional antenna and directs radio frequency (RF) energy at the ground (terrain), the energy then being reflected from the ground into the radar receiver of an adversary through the main lobe of the radar antenna. This presents a problem when the radar, while searching for a target when in a look down situation, has its main radar beam directed at the terrain which also has the jamming signal directed thereon. The clutter caused by the jamming signal which is reradiated from the terrain to the radar, when received through the main beam, adds with normal receiver noise to increase the noise floor or threshold of the receiver. This jamming technique is also referred to as producing xe2x80x9chot clutterxe2x80x9d. Depending upon the system design, the effect of this type of jamming can be many times worse than what the jammer level would be if the clutter were directed just at the side lobes of the radar. Accordingly, it is necessary to eliminate or at least greatly reduce the effect of hot clutter as above described on a high performance radar system. No prior art is known in connection with the solution to this problem.
In accordance with the present invention, there is provided a system and method for minimizing the effect of jamming of the type discussed hereinabove.
Briefly, a secondary beam is generated by the radar which is slightly offset from the primary data collection beam (sum beam) to obtain a hot clutter reference which is used in a conventional side lobe jammer processing algorithm, preferably a Gram-Schmidt algorithm. Advantage is taken of the typical geometries of a standoff jammer which provides nearly matched dispersion characteristics of the jammer RF when viewed with the two received beams. The important parameter is dispersion relative to the received bandwidth. The secondary beam, which may be an identical copy of the main sum beam, is offset from the primary beam by approximately two beam widths toward the jammer. The offset distance is chosen so that there is minimum and preferably no overlap in the main beam footprint on the ground and that of the secondary beam, but the footprints of the two beams are as close together as possible. If there is overlap, then there is a chance that the target will appear in both beams and therefore be suppressed. Accordingly, overlap must be minimized.
The signal processing hardware is configured with a conventional Gram-Schmidt algorithm with tapped delay lines feeding in the auxiliary ports or other appropriate cancellation algorithm. The number of auxiliary ports that are required depends upon the receiver bandwidth, the geometries of the jammer and the reflecting surface. The number of taps required is proportional to the receiver bandwidth.
Implementation requires the capability of generating the offset secondary beam. This is accomplished by adding another azimuth manifold with a slight phase offset. Alternatively, another set of phase shifters in azimuth can be used with another feed tied on the output of the phase shifters, allowing one to place the secondary beam between the jammer and the main beam independent of the pointing of the main beam relative to the jammer. The receiver requires a separate auxiliary channel and analog to digital converters and the processor requires additional taps to its jammer canceler processor. Typically, this requires adding taps but not a unique algorithm to process this data. An alternate approach to generating the azimuth beam is generating an auxiliary beam that can be pointed at the jammer itself. The advantage of this approach is to provide a better jammer to noise ratio. The disadvantage is that the jammer can point a spoofing signal or a signal that is at a different frequency of different noise content toward the receiver, thus the receiver would have the wrong canceling information. This alternate approach also requires a second set of phase shifters throughout the antenna array, increasing system cost and complexity. This reflective jammer canceler approach can also be applied to self protect reflective jamming. The geometries are typically worse from a dispersion view point but will still provide jammer suppression. Again, the amount of suppression is dependent upon exact geometries, receiver bandwidth and the number of delay line taps going into the jammer canceler.
Additional auxiliary beams can be used to provide improved performance.
In summary, this invention provides a method to greatly decrease the impact of a very serious ECM threat.